Examples of some hot melt applicator systems are disclosed in the Baker, et al., U.S. Pat. Nos. 3,690,518 and 3,840,158, as well as in Frates et al., U.S. Pat. No. 4,579,255, all of which are assigned to the assignee of the present invention. In these and some prior art hot melt applicator systems, the qualitative responsiveness of the system in terms of the applied bead consistency, bead width, bead placement, bead edge quality, etc. may deteriorate at the end of the bead.
In order to provide further precision to the adhesive dispensing process, the Lewis, et al. U.S. Pat. No. 4,801,051 which is assigned to the assignee of the present invention, discloses a similar fluid dispensing valve in which a new valve stem guide is used. In addition, a device for fine adjustment of the maximum travel of the valve stem accurately and adjustably controls the flow of liquid through the nozzle opening. While this design improved the performance of the adhesive dispensing valve in certain applications, some adhesive continues to collect in the dispensing channel after valve closure.
With the above described systems, the valve seat, discharge orifice, and dispensing channel therebetween are all an integral part of the nozzle body, which is mounted with fasteners to the valve operating module. Consequently, with this and some other prior art systems, if it is desired to change the size of the discharge orifice, or if the orifice becomes clogged, it is necessary to remove the fasteners and the entire nozzle body in order to flush the system and manually clean the discharge channel and orifice only after the fluid pressure of the hot melt adhesive has been removed from the dispenser. If the adhesive being dispensed is a hot melt adhesive, the adhesive will generally be maintained at a temperature within the range of about 250.degree. F. to about 425.degree. F.; and therefore, the handling of hot valve components on disassembly and flushing the valve with the hot melt adhesive must be done very carefully.
In addition, after the valve is cleaned, it is cold and reassembling the cold nozzle body to the valve operating module, which contains the hot melt adhesive, will result in a premature hardening of the adhesive upon its initial contact with the cold nozzle body. Such cooling increases the risk of clogging of the dispensing valve. To avoid that premature cooling, auxiliary heating elements or heat guns are used to heat the cold nozzle body and the adhesive in contact therewith. Consequently, there is a disadvantage with the above in that the process of changing and cleaning the dispensing nozzle is complicated and may shut down a production line for more than one hour.
There are nozzle designs in which a nozzle plate containing the discharge orifice is secured to a valve by a mounting nut such as that shown in Vilagi et al. U.S. Pat. No. 4,360,132, assigned to the assignee of the present invention. However, none of the nozzle plates that are held on with a mounting nut and can be quickly removed contain the dispensing valve seat and its connecting dispensing channel. Therefore, with those designs, the valve seat and the dispensing channel cannot be readily cleaned or exchanged without disassembling of the dispensing valve.
Further, even though the dispensing channel in newer valve designs is to a great extent self-cleaning, small amounts of adhesive may still remain in the dispensing channel after the valve is closed. This remaining adhesive may harden and form one or more small chips or particles which may adversely affect subsequent dispensing cycles. For example, during the start of a subsequent cycle, the trajectory of those particles of adhesive is unknown and unpredictable. Further, the hardened particles may stay in the dispensing channel and deflect a subsequent adhesive stream. Consequently, all of the above designs have the disadvantage that some adhesive remains in the dispensing channel and is not subject to adhesive dispensing process control.
In the above designs, the valve seat, the dispensing channel, and the discharge orifice are all located at one end of the relatively long and narrow nozzle body and must be machined by obtaining access through the opposite end of the centrally located and relatively narrow adhesive cavity within the nozzle body. A disadvantage of those designs is that the machining of the valve seat, dispensing channel and discharge orifice is a complex and expensive process.
Finally, in some applications, newer adhesive formulations are more chemically aggressive and corrosive than previous adhesives. Further, the corrosion resistant materials from which the adhesive dispensing valve must be made are typically more exotic or expensive and more difficult to manufacture. This may require that the whole nozzle body, including the nozzle section, must be made from the more expensive material if it is physically or economically feasible.